We use "Fracture-permeation" to study the compactness of myofilaments within the sarcomere during muscle contraction. As models of skeletal muscle, we utilize sartorius muscle from toad (Bufo marinus) and papillary muscle from the left vantriale of Sprague-Dawley rats. Tissue fixed in glutaraldehyde was frozen, cross-fractured in liquid nitrogen and thawed. Tissue fragments were immersed in concentrated solutions of native ferritin (30% w/v). Permeation by ferritin, an electron-dense probe, tested the existence and distribution of intermolecular spaces within the sarcomere of glutaraldehyde-fixed muscle cells. Our results lead to the first unequivocal ultrastructural characterization of the contracted stage in cardiac muscle cells. Qualitatively distinct patterns of ferritin permeation into the sarcomere lead to immediate identification of all stages of muscle contraction. Macromolecular permeation of freeze-fractured skeletal muscle characterizes and distinguishes rigor, contracted and relaxed states. Morphological identification of contracted muscle and the pattern of permeation by ferritin into sarcomere at this state raises questions concerning the molecular mechanisms of muscle contraction. These new results have reinforced our expectations on "Fracture-permeation" as a new approach to study intracellular matrices.